The present invention relates to a method of controlling the state of charge of an electric energy accumulator (i.e. an energy store), particularly a battery, of a hybrid vehicle, which has an internal-combustion engine and an electric machine, the battery being operated in a working state-of-charge range (30%-80%), which is completely within the theoretically possible state-of-charge range (0%-100%) of the battery.
So-called hybrid vehicles are characterized in that, in the coasting operation of the vehicle, kinetic energy is recuperated by way of an electric machine, i.e., converted to electric energy and stored in an energy accumulator. In driving phases of the vehicle, the stored energy can be used for operating the electric machine as an electric motor and/or for supplying electrical energy to the diverse electrical vehicle systems.
It is an object of the invention to control the state of charge (SoC) of the electric energy accumulator such that a long useful life of the energy accumulator is obtained.
This and other objects are achieved by a method of controlling the state of charge of an electric energy accumulator, particularly a battery, of a hybrid vehicle that has an internal-combustion engine and an electric machine. The electric energy accumulator is operated in a “working state-of-charge range” that is within the theoretically possible state-of-charge range of the energy accumulator. Theoretically, the energy accumulator can take up a state of charge in the range of between 0% (energy accumulator is empty) and 100% (energy accumulator is fully charged). Tests have shown that, if possible, the state of charge of the energy accumulator in operation should not be below a predefined lower limit and, if possible, also not above a predefined upper limit because this would otherwise considerably decrease the useful life of the energy accumulator. The “working state-of-charge range” may, for example, be defined as the state-of-charge range between 30% and 80% of the maximal state of charge.
It is an aspect of the invention that the working state-of-charge range is functionally divided into four sub-ranges, which can be characterized as follows.
In a first sub-range (SoC is low) of the working state-of charge range, the battery is charged by the electric machine of the hybrid vehicle. The electric machine therefore works as a generator and, in the process, is driven by the internal-combustion engine of the hybrid vehicle.
In a second sub-range (SoC low optimum) of the working state-of-charge range, the battery is neither charged nor discharged. Also in this sub-range, the electric machine operates as a generator and is driven by the internal-combustion engine. However, the electric power generated by the electric machine is just high enough for supplying the on-board power system of the hybrid vehicle with current.
In a third sub-range (SoC upper optimum) of the working state-of-charge-range, the battery is discharged, in which case the battery supplies the on-board power system of the hybrid vehicle with current. In the third sub-range of the working state-of-charge range, the electric machine is running along passively. “Running along passively” means that it neither receives electric power, nor delivers electric power. In the third sub-range, the recuperation energy is utilized more efficiently in that the existing recuperated stored energy is utilized for supplying the electrical consuming devices present in the vehicle.
In a fourth sub-range (SoC is high) of the working state-of-charge range, the battery is being discharged. In this fourth sub-range, the electric machine of the hybrid vehicle operates as an electric motor and is supplied with current by the battery.
According to a further aspect of the invention, the working state-of-charge range has a lower limit which is in the range of between 25% and 35%, particularly at approximately 30% of the maximal state of charge. The upper limit of the state of charge range may be in the range of between 75% and 85%, particularly at approximately 80% of the maximal state of charge.
The first sub-range of the working state-of-charge range is defined by the lower limit of the working state-of-charge range and a first sub-range limit. The first sub-range limit may, for example, be in the range of between 50% and 55% of the maximal state of charge of the energy accumulator. The first sub-range may therefore, for example, be between 30% and 55% of the maximal state of charge of the energy accumulator.
The second sub-range of the working state-of-charge range is defined by the first sub-range limit and a second sub-range limit, which may, for example, be at 60%.
The third sub-range of the working state-of-charge range is defined by the second sub-range limit and a third sub-range limit which may, for example, be at 65% of the maximal state of charge of the energy accumulator.
The fourth sub-range of the working state-of-charge range is defined by the third sub-range limit and the upper limit of the working state-of-charge range. The fourth sub-range may therefore, for example, be between 65% and 80% of the maximal state-of-charge of the energy accumulator. Therefore, in the first sub-range a “load point elevation” of the internal-combustion engine takes place, which means that the electric machine is operating as a generator and represents an additional load for the internal-combustion engine. In contrast, the fourth sub-range is characterized by a load point drop, which means that the electric machine is operating as an electric motor and aids the vehicle drive or the internal-combustion engine.
According to another aspect of the invention, the transitions between the individual sub-ranges of the working state-of-charge range are constant (in a mathematical sense). They can, for example, be defined by ramp functions. In other words, the charging moment of the electric machine has a constant course over the state of charge. However, the course must not necessarily be constantly differentiable. The deflection of the charging moment of the electric machine after the state of charge may rather have an unsteady course, for example, a course that is constant in sections.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.